1. Field of the Invention
The present invention relates to data storage devices and, in particular, to suppression of mechanical resonance, and more particularly to suppression of mechanical resonance by using a notch filter.
2. Description of the Related Art
A hard disk drive widely used as an external storage device for a computer device is provided with a magnetic head for reading user data stored on a magnetic disk, or for writing user data on the magnetic disk. The magnetic head is attached to an actuator mechanism oscillated by a voice coil motor (VCM). When reading or writing of the user data is executed by the magnetic head, the actuator mechanism is driven to move and position the magnetic head on a specific track (target track). The magnetic head is controlled to move to a predetermined position based on servo information stored on the magnetic disk.
In the magnetic disk such as a hard disk, a plurality of data tracks are concentrically formed, and a servo track containing identification information and a burst pattern prestored therein is formed along a radial direction of the magnetic disk. The identification information and the burst pattern constitute the aforementioned servo information. The identification information indicates a track address of each data track. Based on the identification information read by the magnetic head, determination can be made as to an approximate position of the magnetic head, i.e., which of the data tracks the position of the magnetic head corresponds to. The burst pattern is made up of a plurality of burst pattern rows in which areas storing signals are arrayed at fixed intervals along the radial direction of the magnetic disk and phases of the signal storage areas are different from one another. Based on a signal (position error signal: PES) outputted from the magnetic head in accordance with the burst pattern, detection can be made as to a precise position of the magnetic head, i.e., deviation regarding how far the position of the magnetic head deviates from the corresponding data track.
The reading/writing of the user data with respect to the magnetic disk is executed in a rotating state of the magnetic disk, after the magnetic head is moved to correspond to a target track while the approximate position of the magnetic head is determined based on the identification information read by the magnetic head, and then the magnetic head is precisely positioned on the target track based on a signal outputted from the magnetic head in accordance with a burst pattern. This series of control is referred to as seek control. Additionally, even during the reading/writing of the user data, feedback control is executed for the target track based on PES so that the magnetic head can follow. This series of control is referred to as following control. These controls are examples of so-called servo control.
In such servo control, for example, when a mechanism of an actuator or the like mechanically resonates at a specific resonance frequency, amplitude of the resonance frequency is superposed on the PES, consequently creating a problem of instability of a seeking operation or a following operation of the magnetic head.
Thus, in the conventional hard disk drive, a notch filter of a center frequency equal to a resonance frequency is inserted into a servo loop, and a gain of the resonance frequency contained in a control signal is reduced by the notch filter, whereby the seeking operation and the following operation are stabilized.
However, there are a subtle individual differences among actuators, and the resonance frequency varies from actuator to actuator. Additionally, in the same actuator, the resonance frequency changes according to a temperature change. Now, if a resonance frequency of an actuator is markedly different from a cutoff frequency band around a center frequency set in the notch filter, abnormalities are exposed by inspection at the time of shipping, thus creating no problems. However, if the resonance frequency of the actuator deviates only slightly from the cutoff frequency band of the notch filter, it is difficult to discover abnormalities by inspection at the time of shipping, creating a concern of problem discovery after shipping. Recently in particular, because of narrow setting of a data track width so as to increase recording density, a servo band frequency must be increased so as to suppress effects of low frequency external disturbances, and effects of mechanical resonance at a relatively high frequency are conspicuous.